Chassis Dynamometers 101 - Ultimate Racing

Did the car actually feel slower coming off Turn 2 or did the motor just bog for a second? The car ahead seemed to pull away coming out of the corner, but maybe you didn’t roll into the throttle fast enough. Although, now that you think about it, your crew chief did make some carb adjustments for that new track you just raced at, so maybe the mixture is a little rich. Of course, it’s also possible that you never got the timing just perfect after switching to that new distributor. Maybe that’s why the car felt “flat” out of 4. Or did it? The other racers just look a bit “snappier” coming on to the back straight. How long has this motor been in the car anyway? Maybe it’s time for a refresh but, then again, it seemed to pull well on the top end.

We’ve all done this. But in this day and age, there is absolutely no reason to be sitting in the pits wondering how well your engine is running or how much power may or may not be left in the tune-up. And, the best part is, you don’t have to track test four days a week or pull the motor every race to get the data you need to make the big show on Saturday night. What are we talking about? Chassis dynamometers and these cool data acquisition devices aren’t just for big-dollar race teams anymore. In fact, you’ve probably got a chassis dyno in your town that you can rent for as little as 75 bucks an hour, and you can make and measure changes to your car without having to guess or “feel” the results.

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Chassis Dyno

A chassis dyno, in the simplest of terms, is a device that measures horsepower and torque at the rear wheels of a car. To fully understand how a dyno works, we asked the DynoJet engineering team directly, who build and produce one of the most well-known and respected chassis dynamometers in the United States. Prepare for some math… “Power, in mechanical terms, is the ability to accomplish a specified amount of work in a given amount of time. By definition, 1 horsepower is equal to applying a 550-pound force through a distance of 1 foot in 1 second. In everyday terms, it would take 1 hp to raise a 550-pound weight up 1 foot in 1 second.

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So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Dynojet’s inertia dynamometer measures power just in this way. The dyno calculates velocity by measuring the time it takes to rotate the heavy steel drum one turn. The dyno measures force at the surface of the drum by indirectly measuring the drum’s acceleration. Acceleration is simply the difference in velocity at the surface of the drum from one revolution to the next. The force applied to the drum is calculated from acceleration using Newton’s Second Law, F=MA, (F)orce equals (M)ass times (A)cceleration. Power is coupled to the drum by friction developed between the driving tire of the vehicle and the knurled steel surface on the drum of the dynamometer.”

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"In this day and age, there is absolutely no reason to be sitting in the pits wondering how well your engine is running or how much power may or may not be left in the tune-up"

5/10Your carb is an extremely important part of your set up. But how do you know if it’s right? With a chassis dyno you can know for sure. The graph shown is from a Super Late Model. From one run to the next, the only change was 2 jet sized to get the air/fuel curve a little leaner. The end result was about 10 horsepower and 10 lb-ft of torque. The gains were not just at the peak, but through almost the entire rpm band. This directly equates to more speed on track!

And that gives us horsepower, which is great, but how do we read torque? “When an object rotates around a point, the object’s speed of rotation depends on both an applied force and the moment arm. The moment arm is the distance from the point of rotation to where the force is being applied. Torque is the product of the force and the moment arm. For example think about trying to spin a drum by wrapping a rope around the drum and then pulling on the rope. If the rope is wrapped around a drum of 1-foot radius and pulled with 550 pounds of force, the resulting torque is 550 foot-pounds. The torque on the dyno’s drum can be calculated by multiplying the force applied by the drum’s radius. However, engine torque is not equal to the dyno’s drum torque because the gearing through the drive train changes the moment arm. The change in the moment arm is proportional to the ratio of engine speed to drum speed. Therefore, tachometer readings are necessary to calculate and display engine torque.”

Got it? Good. If not, just try to think of a chassis dyno as a rolling road, a torque wrench, and an engine dyno all wrapped into one device. It’s a racetrack that stays still, and it will allow you to get precise, accurate horsepower and torque data from one run to the next, allowing you to make and measure small changes in your tune-up and combination. Like an engine dyno, you’ll get an abundance of useable information, but unlike an engine dyno, you will also learn about the rest of your drivetrain in the process. And, worry not, because a quality chassis dyno will be as reliable and repeatable as you need it to be, given that everything on your vehicle stays the same. Which brings us to…

Consistency

6/10Spark issues are also not always easy to diagnose. This dyno graph shows the example of a Late Model with a bad distributor. The spark wasn’t consistent and the car never ran clean. The team changes carbs thinking it was a fuel issue, and swapped ignition boxes thinking that was the problem, but it never went away. After chasing the problem for the better half a season, a day on the chassis dyno helped them figure out the distributor was bad, and that weekend they ran up front.

If you are looking to test engine modifications, such as timing changes, carburetor tune-ups, or anything else related to horsepower and torque production, it is important to remember to keep everything else on the car consistent in order to achieve reliable, repeatable, and accurate results. Said another way, you only want to change one thing at a time on the chassis dyno, and you want to ensure that no other variables are influencing your results. If your objective is to test timing curve changes then that should be the only thing changed between runs. Do not, and we repeat -- do not -- make several changes at once and hope for the best. One change may add 10 rwhp and the other may lose 6, but you’ll only see a 4-rwhp difference on the graph. And then you’ll just be guessing.

It is important to remember to keep everything else on the car consistent in order to achieve reliable, repeatable, and accurate results.

As important as it is to only make one change at a time, it is also important to remember to keep other variables out of the equation. That means tire pressure must remain consistent from dyno run to dyno run and dyno session to dyno session. Additionally, you want to strap the car down to the dyno using the same mounting points and the same tension on each strap.

7/10Electrical gremlins can also be hard to diagnose. When you race wheel to wheel and trading paint is the norm, cut wires and shorts can happen. This Mini Stock battled a problem with falling off hard down the straights. You can see on the dyno graph that power and torque fall off dramatically. The car runs stock ignition, which is required in the class he runs. A crash cause two partially cut wires for the ignition that were in the harness and impossible to see. Once the wires were repaired, the car made good power and was back to normal on track.

Engine coolant temperatures, along with the transmission and rear end fluid temperatures should also be maintained, as heating or cooling the fluids will affect horsepower and torque readings. Obviously, we can’t control the weather, and the dyno will compensate for changes in the weather automatically, but you should always attempt to make changes while the weather is normalized. A run at 6:30 in the morning with no humidity and cool temperatures, compared to one after heat soaking the engine, the dyno cell, and the drivetrain done at high-noon with high humidity and 20-degree higher temperatures will not be very accurate for measuring small changes.

Real World Applications:

Ok, so we know how a dyno works and we understand that we must be consistent, but how can we use one of these tools in a meaningful way? Let’s look at a couple of real world scenarios:
Timing, timing, timing

8/10Sometimes the difference between winning and losing can be nothing more than a degree of timing. This graph shows how beneficial knowing how timing changes actually affect power. At the peak, a few degrees of timing can be worth as much as 10 horsepower!

Your car runs great at the track, and your starting to push into the front of the pack. The guys up front always snap off of the corners, while you’re thinking your car might be a little lazy coming into the power band. And, try as you might, it’s hard to get your foot any further through the floor. You’ve heard people talk about the effects of ignition timing advance and its affect on power but you really can’t afford to take each heat race and qualifying run with plus or minus half a degree of timing. On a chassis dyno however, you can quickly change timing in seconds, and you can record the entire rpm range and the affect the timing change made. With 28 degrees of total timing, you may make peak torque of 500 lb-ft at 4,700 rpm. A quick bump to 30 degrees total, and you gained 10 lb-ft and brought peak torque down a bit to 4,600 rpm. If some is good, more must be better, right? Add 32 and, uh oh, peak torque has dropped off and you’ve picked up a little pre-detonation, which caused erratic power output above 5,000 rpm. Now, in just three runs, you’ve found the best timing for maximum performance and the testing is backed by actually hard data.

"You’ve heard people talk about the effects of ignition timing advance and its affect on power but you really can’t afford to take each heat race and qualifying run with plus or minus half a degree of timing."

Carbs for breakfastA bigger carb is always better for power, everyone knows that, right? No, you know better than that, but maybe a small change in flow could really help your particular combination. Now, are we going to go to the track with six different carbs and test them all? What about traffic, or the track surface changing throughout the night? Again, load that racecar up to the chassis dyno, stabilize all of your variables (tire pressure, straps, temps, and so on) and let’s change out some fuel leakers. Run one, record the data, remove it, and drop another carb in its place. Oh look, the smaller carb actually made much more power under the curve, while the big fancy one actually dropped power by 25 rwhp across the top of the curve. The big guy may have “felt” better, but here again, we have actual data to prove otherwise.

9/10Is your car hard to push around in and out of the shop? If you car doesn’t roll well, that’s most likely cause there is some sort of bind condition happening somewhere in the drivetrain. Being in bind does nothing but rob power. If it takes more power to turn the wheels, it’s less power getting to the ground. The graph above is from a Street Stock team that had a Monte Carlo that was very hard to push. The driveline was misaligned and the rearend was out of square. Because of this, the car fought itself just to roll. Freeing all the bind up was like finding free horsepower for this team.

Clutch Situation Remember when we said that the chassis dyno measures everything in the drivetrain, not just the engine? Well, that really means that everything can be measured for power gains or losses, including all of the hard parts between the engine and rear tires, and even some accessories we wouldn’t normally consider. Running an alternator? Ever wonder if a looser belt may free up a pony or two? Or, maybe that heavy clutch really is soaking up some power, what would a lighter one do? How about the actual gear oil in the rearend? Think you could go to a lighter weight oil and pick up some power in the process?

Testing, Testing, 1, 2, 3…

Are you starting to see just how much information you can gather with a tool like this? The big NASCAR teams have been doing it for years, and now you can get in on the action for less than it costs to tow your rig to some races. How about those low-impedance plug wires you keep hearing about? The list goes on and on and the only way to know what really works on your exact combination is to try it out and see for yourself. So, head on down to your local dyno shop and start making pulls. You’ll find what works, you’ll find what doesn’t, and you may even find some new tricks you’d never even thought of. At the end of the day, you’ll be more prepared come race day and you will have quality data, which can be the difference between guessing as to why you lost or climbing atop your car and holding up that big track trophy.

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At the end of the day, you’ll be more prepared come race day and you will have quality data, which can be the difference between guessing as to why you lost or climbing atop your car and holding up that big track trophy.